Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers ("heads") mounted to a radial actuator for movement of the heads relative to the discs.
Typically, such radial actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from an actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit comprising one or more permanent magnets and magnetically permeable pole pieces. When controlled DC current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces.
Control of the position of the heads is typically achieved with a closed loop servo system such as disclosed in U.S. Pat. No. 5,262,907 entitled HARD DISC DRIVE WITH IMPROVED SERVO SYSTEM, issued to Duffy et al., assigned to the assignee of the present invention. A typical servo system utilizes servo information (written to the discs during the disc drive manufacturing process) to detect and control the position of the heads through the generation of a position error signal (PES) which is indicative of the position of the head with respect to a selected track. More particularly, the PES is typically presented as a position dependent signal having a magnitude indicative of the relative distance between the head and the center of a track and a polarity indicative of the direction of the head with respect to the track center. Thus, it is common for the PES to have normalized values corresponding to a range of, for example -1.0 to +1.0, as the head is swept across a selected track and to have a value corresponding to a value of 0 when the head is positioned over the center of the track. As will be recognized, modern servo systems typically generate the PES as a sequence of digital samples which generally correspond to the above analog range.
The PES is generated by the servo system by comparing the relative signal strengths of burst signals generated from precisely located magnetized servo fields in the servo information on the disc surface. The servo fields are generally arranged in an offset pattern so that, through manipulation of the magnitudes of the burst signals provided to the servo system as the servo fields are read, the relative position of the head to a particular track center can be determined and controlled. More particularly, digital representations of the analog burst signals are typically provided to a servo loop microprocessor (or digital signal processor), which obtains a digital representation of the value of the PES from a selected combination of the input digital representations of the analog burst signals. The microprocessor then compares the value of the PES to a desired value indicative of the desired position of the head to the selected track and issues a digital correction signal to the power amplifier, which in turn provides an analog current to the actuator coil to adjust the position of the actuator.
It will be recognized that accurate control of the position of the heads is of paramount importance in the reliable reading and writing of data to the discs. The servo loop generally attempts to maintain the head over the center of the selected track so as to minimize the potential for overwriting data on adjacent tracks or having the magnetization of adjacent tracks interfere with the reading of the data stored on the selected track. Thus, it is common during read and write operations to compare the absolute value of each PES sample to a predetermined safe-threshold value in order to assure the head is correctly positioned relative to the track. Should the value of the PES for a particular sample exceed the threshold, the read or write operation is temporarily suspended until the PES is brought back down to a safe value.
A selected PES sample may have a value that exceeds the safe-threshold value during a read or write operation for a variety of reasons. One such reason is that the head is actually positioned off track center a distance sufficient to exceed the threshold value; particularly, it will be recognized that mechanical shocks supplied to the disc drive during operation can result in movement of the head away from the center of the selected track (sometimes referred to as an off-track condition). As a result, it is desirable to suspend the read or write operation until such off-track condition can be corrected.
Another reason that a selected PES sample may have a value that exceeds the safe-threshold value is the existence of a localized defect in the servo information associated with the PES sample; in such a case, the head is correctly located with respect to the track, but the reported PES sample erroneously indicates otherwise. Such a defect in the servo information can occur as a result of a localized anomaly in the media on the surface of a disc, so that the media does not possess the necessary magnetic properties to allow the servo information to be written at this location. Additionally, errors can occur during the servo track writing process during manufacture of the disc drive, so that incorrect servo information is provided to the disc at a particular location.
Regardless of the source of the defect in the servo information, such a defect is typically manifested as a one sample error in the PES. The erroneous PES sample does not provide a true indication of head position relative to the center of the selected track, and further, if the erroneous PES sample is interpreted by the servo loop as an impulse function, an unwanted oscillatory response will be induced into the system.
Because of the problems associated with defects in the servo information, it is desirable to provide a servo loop which is capable of determining when an off-track condition is caused by a true positioning problem (and make the necessary position corrections) and when an off-track condition is caused by a servo defect (and ignore the erroneous PES sample). However, prior art servo loops have heretofore been generally unsuccessful in distinguishing excursions in the PES that are caused by servo defects from those caused by external shocks to a drive. This is particularly true in servo loops which rely on the PES value to identify off-track conditions.
The servo information, including the servo fields, are written to the discs during the manufacturing process using a highly precise servo track writer. Although methodologies vary in the writing of the servo information, typically the disc drive is mounted on the servo track writer and the appropriate write signals are provided to the heads of the disc drive to write the servo information while the discs are rotated by the disc drive spindle motor. A mechanical pusher arm is used to incrementally advance the heads over the surfaces of the discs while a closed loop positional control system is used to locate the heads relative to the discs.
Occasionally, a particularly egregious type of defect in the servo information known as a "track tear" can be generated during the servo track write process. A track tear is characterized as a radial discontinuity, so that a track having a track tear ends at a different radius than it begins; that is, the radius of the track varies with respect to angular position over at least a portion of the track. Vibrations, mechanical shocks and errors in the servo track writer position control system can contribute to the generation of a track tear condition.
It will be recognized that a track tear provides a unique situation for a disc drive, in that mapping out and coasting over the servo information at the discontinuity will not resolve the defect, unlike other types of servo defects wherein mapping out and coasting over the defect generally provides adequate compensation for the defect. Mapping out and coasting over a track tear merely "shifts" the defect to the next frame on the track. Whereas a regular servo defect generates what may be characterized as a "spike" in the PES, a track tear generates a "step function" in the PES. The significance of this distinction is that with regard to a track tear, during subsequent operation of the drive the head will continue to be displaced with respect to the track even after the first frame associated with the radial discontinuity of the track tear has been mapped out.
Even though the displacement associated with a track tear can be relatively small, during write operations the disc drive closely monitors the position of the head and will declare a write fault if the head is displaced from the center of the track a small amount, which is typically around 10% of the width of the track. Thus, even a relatively small track tear can result in the repeated declaration of a write fault by the disc drive each time the head attempts to write data proximate to the track tear. Such write faults will not be corrected by mapping out the first frame associated with the track tear, as the write fault will merely be shifted to the next frame immediately following the mapped out frame.
Heretofore, the prior art has not been able to adequately distinguish between servo defects and actual head displacements, and further has not been able to adequately distinguish between regular servo defects and track tear conditions. Accordingly, there is a need for an improved approach to detecting servo defects in general, and more particularly to detect track tear conditions in a disc drive.